4 Answers
4

Wrappers, being objects, get placed in the heap. Primitives are just "values" and go in the stack. This is more efficient, because for wrapped primitives in the heap you need (at least) both the value (which is in the stack) and the reference to the wrapper object.

Whether this performance gain matters at all depends on what you're doing. For heavy numerical work, definitely, but for 99 % of stuff out there, this is rather an annoyance. For one thing, you can't store primitives in a Collection anyway; they get autoboxed. So the only way to store lots of them is to use plain arrays, which in turn can lead to other kinds of inefficiencies (if you need to resize them, for instance).

are there any others reasons than storage?
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GuruKulkiJan 14 '10 at 11:06

I'm not 100% sure about how the internals work here, but I believe that to calculate anything with the numbers, the machinery has to first extract the values from the wrappers (unboxing), which takes at least some work. But the JIT runtime optimization is probably playing its tricks here, so the difference may be quite negligible in practice, even in numerically intensive programs. This should be measured to be sure.
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Joonas PulakkaJan 14 '10 at 11:10